A variety of detection methods have been proposed to detect the movement of an object within the range of an ultrasonic detection system, but one main practical problem is to provide for relative immunity from ultrasonic interference giving rise to false alarms. Such interference may be caused for example by the changing density of air produced by fan heaters, and this in turn results in a variation of the echo pattern of fixed objects within the range of the detection system. Another source of interference is ultrasonic noise produced for example by a metallic object, such as a spanner, which has been dropped, or by a bell ringing. This practical problem at least partly arises due to difficulties in setting the sensitivity of the detection system.